1. Field of the Invention
The present invention relates to a noncontact tonometer that blows an air current against an eye and measures optically a deformation of its cornea caused by the air current to determine the intraocular tension of the eye by noncontact measurement.
2. Description of the Related Art
Conventionally there is widely known a noncontact tonometer that measures the intraocular tension of an eye by a noncontact measuring method. In the method, first, the eye is blown by an air pulse to be a state of the applanation of its cornea. The state of the applanation means that the top portion of the cornea is deformed to be flat. Hereinafter, the word, "applanation", means the same meaning. The state of the applanation of the cornea, second, is detected optically in the method so that the intraocular tension of the eye is measured.
More specifically, an air pulse Ap whose pressure to the eye increases in proportion to time as shown in FIG. 1 is blown against the eye, and the eye is irradiated with parallel light beams Li as shown in FIG. 2.
A cornea C in an ordinary state has a spherical surface before being blown with an air pulse. Therefore, when the cornea C in the ordinary state is irradiated with the incident light beams Li, reflected light beams Lr travel as if the reflected light beams Lr are radiated from a point A at a distance equal to half the radius of curvature of the cornea C forward along the optical axis of the eye E from the center Ro of curvature of the cornea C.
In a state where the cornea C is flattened as shown in FIG. 3 by the air pulse, the reflected light beams Lr travel in parallel to each other. The reflected light beams Lr are detected by a light receiving device disposed so that the intensity of light fallen thereon is a maximum when the cornea C is flattened. Time T0 (FIG. 1) elapsed before the intensity of received light increases to the maximum, i.e., time elapsed before the cornea C is flattened, is measured, and intraocular tension is calculated on the basis of the time T0.
This noncontact tonometer is unable to measure the intraocular tension of the eye unless the eye is flattened. Therefore, the pressure of the air pulse must inevitably be high enough to achieve the applanation of the eye. The air pulse of such a high pressure is unpleasant to the subject. The air pulse generates a large sound to startle the subject.